1. Field of the Invention
The present invention relates to a gas sensor for measuring oxides such as NO, NO.sub.2, SO.sub.2, CO.sub.2, and H.sub.2 O, and inflammable gases such as CO and CnHm contained, for example, in atmospheric air and exhaust gas discharged from vehicles or automobiles.
2. Description of the Related Art
Various measuring systems and apparatuses have been hitherto suggested in order to know the concentration of a desired gas component contained in a measurement gas.
Those known as methods for measuring NOx in a measurement gas such as combustion gas include, for example, a technique based on the use of the NOx-reducing ability of Rh, in which a sensor comprising a Pt electrode and an Rh electrode formed on an oxygen ion-conductive solid electrolyte such as zirconia is used to measure an electromotive force generated between the both electrodes.
The sensor as described above involves problems in that the electromotive force is greatly changed depending on the change in concentration of oxygen contained in the combustion gas as the measurement gas, as well as in that the change in electromotive force is small with respect to the change in concentration of NOx, and hence the sensor tends to be affected by noise.
On the other hand, in order to induce the NOx-reducing ability, it is indispensable to use a reducing gas such as CO. In general, under a fuel-lean combustion condition in which a large amount of NOx is produced, the amount of produced CO is smaller than the amount of produced NOx. Therefore, the conventional gas sensor has a drawback that it cannot perform measurement for a combustion gas formed under the combustion condition as described above.
Japanese Laid-Open Patent Publication Nos. 63-38154 and 64-39545 disclose a system in which a set of electrochemical pumping cell and sensor cell comprising Pt electrodes and an oxygen ion-conductive solid electrolyte and another set of electrochemical pumping cell and sensor cell comprising Rh electrodes and an oxygen ion-conductive solid electrolyte are combined to measure NOx on the basis of the difference between pumping current values of the respective ones.
Japanese Laid-Open Patent Publication Nos. 1-277751 and 2-1543 suggest a method in which two sets, i.e., two pairs of electrochemical pumping cells and sensor cells are prepared. A sensor, which comprises one set of the pumping cell and the sensor cell, is used to measure a limiting pumping current at a partial pressure of oxygen at which NOx is not reduced. A sensor, which comprises the other set of the pumping cell and the sensor cell, is used to measure a limiting pumping current at a partial pressure of oxygen at which NOx is reduced. The difference between the measured limiting currents is measured. Alternatively, a method is suggested in which the difference in limiting current is measured by using a sensor comprising a set of pumping cell and sensor cell, while switching the partial pressure of oxygen in a measurement gas between a partial pressure of oxygen at which NOx is reduced and a partial pressure of oxygen at which NOx is not reduced.
However, in the foregoing systems for measuring NOx, the greater part of the value of the limiting current is occupied by a current brought about by oxygen which is contained in a large amount, and a current based on the objective NOx is extremely small in ordinary cases. As a result, a small current value corresponding to NOx is determined from a difference between the two large current values. Therefore, in the case of the measuring system based on the switching while using one set of sensor, continuous measurement is unsuccessful in some cases. Further, for example, such a system involves problems in that the response is slow, and the accuracy is inferior.
In the case of the system in which the two sets of sensors are used, an error tends to occur in the measured value when the oxygen concentration in the measurement gas greatly changes. Such a system cannot be used in some cases, for example, for automobiles in which the oxygen concentration in the measurement gas greatly changes. This inconvenience results from the fact that the dependency on oxygen concentration of the pumping current of one sensor is mutually different from the dependency on oxygen concentration of the pumping current of the other sensor.
For example, in the case of an automobile, the oxygen concentration in exhaust gas is several % under a driving condition of an air-fuel ratio of 20, while the NOx concentration is several hundreds ppm. Therefore, NOx has a concentration of about 1/100 of that of oxygen. In this case, only if the dependency on oxygen concentration of the pumping current slightly differs, the difference in limiting current value with respect to the change in oxygen concentration becomes larger than an amount of change in limiting current caused by NOx to be measured.
In addition, the foregoing sensor has had the following inconveniences. Namely, when a diffusion rate-determining means for the pumping cell is clogged with burned products of oil contained in exhaust gas, then a change occurs in the pumping current, and the accuracy is deteriorated. Further, when the temperature of exhaust gas is greatly changed, an abnormal factor arises in the measured value.
Moreover, the system comprising the two sets of sensors has had the following drawback. Namely, if a difference occurs between chronological changes in respective characteristics of the sensors, the difference exactly provides an error, and the system cannot be used for a long period of time.
As described above, oxygen existing in the measurement gas involves various problems on the NOx measurement. Further, oxygen arises similar problems such as decrease in measurement accuracy when measurement gas components other than NOx are measured. It has been strongly demanded to solve these problems.
The present inventors have disclosed, in Japanese Laid-Open Patent Publication No. 8-271476, a new measuring system based on the use of first and second electrochemical pumping cells arranged in series, in which a measurement gas component having bound oxygen such as NOx in a measurement gas can be measured accurately in a continuous manner with good response for a long period of time without being affected by the oxygen concentration or the change thereof in the measurement gas.
The measuring system, which has been illustratively proposed, will be briefly explained especially for its measuring procedure. At first, a measurement gas, which contains a gas component having bound oxygen to be measured, is successively introduced into first and second processing zones under predetermined diffusion resistances respectively from an external measurement gas-existing space.
In the first processing zone, oxygen in the atmosphere is pumped out by using the fist electrochemical pumping cell. Thus the value of partial pressure of oxygen is controlled to be low, at which the measurement for the amount of the objective component is not substantially affected.
In the second processing zone, the measurement gas component in the atmosphere, which has been introduced from the first processing zone, is reduced or decomposed. Oxygen produced during this process is pumped out by the aid of the oxygen-pumping action effected by the second electrochemical pumping cell.
A pumping current, which flows through the second electrochemical pumping cell, is detected to obtain a detected value from which the amount of the objective component in the measurement gas is determined.
However, as a result of further investigations on the illustratively proposed measuring system, the following problem has been revealed. Namely, when the oxygen concentration in the measurement gas is increased, the partial pressure of oxygen in the atmosphere introduced from the first processing zone to the second processing zone is changed (increased), although the oxygen concentration (partial pressure) in the atmosphere in the first processing zone is controlled by adjusting the pumping voltage of the first electrochemical pumping cell so that the electromotive force detected for the first processing zone by an oxygen partial pressure-detecting means (electrochemical sensor cell) has a constant value.
Namely, when the oxygen concentration in exhaust gas increases, the decomposing current for NO in the second processing zone also increases. As a result, it is feared that the improvement in measurement accuracy undergoes a limit.
The foregoing phenomenon is caused as follows. Namely, even when the oxygen concentration in the first processing zone is controlled to be constant by using the first electrochemical pumping cell in the first processing zone, if the oxygen concentration in exhaust gas greatly changes, for example, by a degree of 0 to 20%, then the distribution of oxygen concentration in the first processing zone changes, and the concentration of oxygen which enters the second processing zone changes.
Accordingly, if the pumping ability of the first electrochemical pumping cell is increased in the first processing zone, the change in concentration of oxygen which enters the second processing zone can be decreased. However, such a countermeasure involves various problems.
Specifically, in order to enhance the pumping ability, those conceivable include, for example, a method to enlarge the area of the pumping electrode, and a method to raise the temperature of the pump. However, if the area of the pumping electrode is enlarged, the area (volume) of the first processing zone is necessarily increased. As a result, it is feared that the response is delayed. On the other hand, if the operating temperature of the pump is raised, reduction of the measurement gas component, for example, NOx tends to occur, or decomposition of NO tends to occur on the pumping electrode in the first processing zone. As a result, it is feared that the sensitivity to NO is lowered.